A number of hydrocarbons, especially lower-boiling or “light” hydrocarbons, in hydrocarbon formation fluids or natural gas are known to form hydrates in conjunction with the water present in the system under a variety of conditions—particularly at a combination of lower temperature and higher pressure. The hydrates usually exist in solid forms that are essentially insoluble in the fluid itself. As a result, any solids in a formation or natural gas fluid are at least a nuisance for production, handling and transport of these fluids. It is not uncommon for hydrate solids (or crystals) to cause plugging and/or blockage of pipelines or transfer lines or other conduits, valves and/or safety devices and/or other equipment, resulting in shut-down, loss of production and risk of explosion or unintended release of hydrocarbons into the environment either on land or off-shore. Accordingly, hydrocarbon hydrates have been of substantial interest as well as concern to many industries, particularly the petroleum and natural gas industries.
Hydrocarbon hydrates are clathrates, and are also referred to as inclusion compounds. Clathrates are cage structures formed between a host molecule and a guest molecule. A hydrocarbon hydrate generally is composed of crystals formed by water host molecules surrounding the hydrocarbon guest molecules. The smaller or lower-boiling hydrocarbon molecules, particularly C1 (methane) to C4 hydrocarbons and their mixtures, are more problematic because it is believed that their hydrate or clathrate crystals are easier to form. For instance, it is possible for ethane to form hydrates at as high as 4° C. at a pressure of about 1 MPa. If the pressure is about 3 MPa, ethane hydrates can form at as high a temperature as 14° C. Even certain non-hydrocarbons such as carbon dioxide, nitrogen and hydrogen sulfide are known to form hydrates under the proper conditions.
Species that tend to form hydrates at hydrate forming conditions in the presence of water include lighter or low-boiling, C1-C5, hydrocarbon gases, non-hydrocarbon gases or gas mixtures at ambient conditions. Examples of such gases include, but are not necessarily limited to, methane, ethane, ethylene, acetylene, propane, propylene, methylacetylene, n-butane, isobutane, 1-butene, trans-2-butene, cis-2-butene, isobutene, butene mixtures, isopentane, pentenes, natural gas, carbon dioxide, hydrogen sulfide, nitrogen, oxygen, argon, krypton, xenon, and mixtures thereof. These molecules are also termed hydrate-forming guest molecules herein. Other examples include various natural gas mixtures that are present in many gas and/or oil formations and natural gas liquids (NGL). The hydrocarbons may also comprise other compounds including, but not limited to CO, CO2, COS, hydrogen, hydrogen sulfide (H2S), and other compounds commonly found in gas/oil formations or processing plants, either naturally occurring or used in recovering/processing hydrocarbons from the formation or both, and mixtures thereof.
Generally, there are two broad techniques to overcome or control the hydrocarbon hydrate problems, namely thermodynamic and kinetic. For the thermodynamic approach, there are a number of reported or attempted methods, including water removal, increasing temperature, decreasing pressure, addition of “antifreeze” to the fluid and/or a combination of these. The kinetic approach generally attempts (a) to prevent the smaller hydrocarbon hydrate crystals from agglomerating into larger ones (known in the industry as an anti-agglomerate and abbreviated AA) and/or (b) to inhibit, retard and/or prevent initial hydrocarbon hydrate crystal nucleation; and/or crystal growth (known in the industry as a kinetic hydrate inhibitor and abbreviated KHI). Thermodynamic and kinetic hydrate control methods may be used in conjunction.
Kinetic efforts to control hydrates have included the use of different materials as inhibitors. For instance, onium compounds with at least four carbon substituents are used to inhibit the plugging of conduits by gas hydrates. Additives such as polymers with lactam rings have also been employed to control clathrate hydrates in fluid systems. These kinetic inhibitors are commonly labeled Low Dosage Hydrate Inhibitors (LDHI) in the art because they may be effectively used to inhibit hydrate formation at dosage levels relatively lower than other inhibitors. KHIs and even LDHIs are relatively expensive materials, and it is always advantageous to determine ways of lowering the usage levels of these hydrate inhibitors while maintaining effective hydrate inhibition.
Another particularly useful group of hydrate inhibitors include dendrimeric compounds and in particular hyperbranched polyester amides. Dendrimeric compounds are in essence three-dimensional, highly branched oligomeric or polymeric molecules comprising a core, a number of branching generations and an external surface composed of end groups. A branching generation is composed of structural units which are bound radially to the core or to the structural units of a previous generation and which extend outward from the core. The structural units may have at least two reactive monofunctional groups and/or at least one monofunctional group and one multifunctional group. The term “multifunctional” is understood as having a functionality of about 2 or higher. To each functionality a new structural unit may be linked, a higher branching generation being produced as a result. The structural units may be the same for each successive generation but they can also be different. The degree of branching of a particular generation present in a dendrimeric compound is defined as the ratio between the number of branchings present and the maximum number of branchings possible in a completely branched dendrimer of the same generation. The term “functional end groups” of a dendrimeric compound refers to those reactive groups which form part of the external surface. Branchings may occur with greater or lesser regularity and the branchings at the surface may belong to different generations depending on the level of control exercised during synthesis. Dendrimeric compounds may have defects in the branching structure, may also be branched asymmetrically or have an incomplete degree of branching in which case the dendrimeric compound is said to contain both functional groups and functional end groups. In one non-limiting embodiment herein, the term “highly branched” may refer to three-dimensional structures that contain a combination of at least 5 functional groups and/or at least 5 functional end groups. Alternatively or in addition to these parameters, “highly branched” dendrimeric compounds may have a number average molecular weight in the range of from about 1000 to about 5000, with a molecular weight distribution of as broad as about 2 to about 30.
Dendrimeric compounds have also been referred to as “starburst conjugates”. Such compounds are described as being polymers characterized by regular dendrimeric (tree-like) branching with radial symmetry.
Functionalized dendrimeric compounds are characterized by one or more of the reactive functional groups present in the dendrimeric compounds having been allowed to react with active moieties different from those featured in the structural units of the starting dendrimeric compounds. These moieties can be selectively chosen such that, with regard to its ability to prevent the growth or agglomeration of hydrate crystals, the functionalized dendrimeric compound out performs the dendrimeric compound. All of these LDHIs are more fully described in U.S. Pat. No. 6,905,605 which is incorporated by reference herein in its entirety.
In addition to dendrimeric oligomers or polymers, suitable gas hydrate inhibitors also include linear polymers and copolymers, such as polymers and copolymers of vinylcaprolactam and/or vinylpyrrolidone, or “onium” compounds such as tetrabutylammonium bromide. Acceptable onium compounds include those mentioned in U.S. Patent Application Publication 2005/0261529 A1, incorporated by reference herein in its entirety.
Hydrate inhibitors are injected into flow lines of produced hydrocarbons, such as oil and gas, that come from subsea wells to prevent the formation of hydrates as the hydrocarbons are being transported to other operations, such as a production facility, the hydrate inhibitors stay with the aqueous phase of these streams unless they are subsequently separated out. These compositions are particularly useful for inhibiting, retarding, mitigating, reducing, controlling and/or delaying formation of hydrocarbon hydrates or agglomerates of hydrates in fluids, particularly those used in hydrocarbon recovery operations. The method may be applied to prevent or reduce or mitigate plugging of annular spaces, pipes, transfer lines, valves, and other conduits, and places or equipment downhole where hydro-carbon hydrate solids may form under conditions conducive to their formation or agglomeration.
The term “inhibiting” is used herein in a broad and general sense to mean any improvement in preventing, controlling, delaying, reducing or mitigating the formation, growth and/or agglomeration of hydrocarbon hydrates, particularly light hydrocarbon gas hydrates in any manner, including, but not limited to kinetically, thermodynamically, by dissolution, by breaking up, by anti-agglomeration other mechanisms, or any combination thereof. Although the term “inhibiting” is not intended to be restricted to the complete cessation of gas hydrate formation, it may include the possibility that formation of any gas hydrate is entirely prevented.
The terms “formation” or “forming” relating to hydrates are used herein in a broad and general manner to include, but are not limited to, any formation of hydrate solids from water and hydrocarbon(s) or hydrocarbon and non-hydrocarbon gas(es), growth of hydrate solids, agglomeration of hydrates, accumulation of hydrates on surfaces, any deterioration of hydrate solids plugging or other problems in a system and combinations thereof.
The term “low dosage” used with respect to low dosage hydrate inhibitors (LDHIs) as defined herein refers to volumes of less than 5 volume % (vol %) of the fluids susceptible to hydrate formation. In some non-limiting embodiments, the vol % for thermodynamic hydrate inhibitors may be considerably higher, which depends on both the system sub-cooling and hold time.
As noted, common KHIs and LDHIs are polymeric, including, but not necessarily limited to, HYBRANE® hyperbranched polymers available from DSM Hybrane, polyvinylcaprolactam (PVCap), polyvinylpyrrolidone, poly(vinylcaprolactam-co-vinylpyrrolidone), polyisopropylmethacrylamide, poly(N-vinyl-N-methylacetamide) (VIMA), poly(VIMA:VCap) copolymer, poly(isobutylacrylamide), hydroxy-ethyl cellulose and its derivatives, and mixtures thereof. Even though these KHIs have relatively low molecular weights, they are typically introduced into the fluids being treated in a solvent, such as monoethylene glycol (MEG), butyl glycol ether (BGE) and methanol (MeOH). These polymeric KHIs have shown some complications in aqueous phase at elevated temperatures, for instance, greater than 100° F. (38° C.), specifically, they tend to precipitate as solids which potentially present plugging problems.
In disposing of produced water in a subterranean aquifer, such as to ultimate dispose of waste water, it is generally assumed that a large amount of water is already present in the formation. Trying to re-solubilize already-formed precipitates, such as by using MEG, methanol (MeOH) or BGE would be expected to merely removing the polymer precipitates from the periphery of or the outside of the formation only temporarily, since polar solvents would play a role in preventing precipitating of the polymeric KHIs only when they are present in relatively high percentages of the aqueous phase. When they contact more water and are diluted further inside the aquifer, the polymeric KHIs would again precipitate out of the solvents and potentially block the formation, preventing further produced water from being introduced.
It is thus desirable to discover methods and compositions for inhibiting the formation of precipitates in produced water that is stored or disposed of.